The analysis of light emission or luminescence from a material is a wide-spread method for determining material properties such as impurity levels, crystal defects, in the field of semiconductors and other materials.
Some methods for generating luminescence include electroluminescence or photoluminescence. The free carriers (electrons and holes) are generated by applying electricity to the material, or generated by applying an excitation light source such as that of a laser, both resulting in light emission via the recombination of the electron-hole pairs.
The electron-hole pairs recombine spontaneously by some probabilistic time after their generation (either electrical or by light excitation). The typical time required for the process to decay is called decay time, or carrier lifetime. The carrier lifetime, as well as other characteristics of the emitted light (for example wavelength, intensity) can be used to determine the condition of the material by means of their dependence on impurities and defects inside the crystal.
In recent years, there has been a large increase in the demand for high efficiency solar-electric cells based on silicon, requiring better quality control and lower fabrication costs, as well as an increase in demand for larger area cells, hence the importance for high throughput fabrication methods of high quality large area silicon crystals.
The reduction of cracks and defects in the material used to make solar cells is crucial. Such cracks and defects absorb the electrical energy that the cell has converted from optical energy. In other words, the whole purpose of the solar cell, to generate electrical energy from light is wasted if the carriers of this energy encounter cracks which then converts this energy into heat. As explained until now, methods such as photoluminescence or electroluminescence are deeply related to the operation mechanism of solar cells. The characteristics obtained from these measurements, particularly the carrier lifetime, are also very sensitive to cracks, the types and level of impurity contents.
The conventional methods for analyzing solar cell efficiency are based on electrical biasing measurement methods, solar light simulation and others requiring physical contact with the device. Also these methods require the fabrication process to reach near completion before they can be performed. It is difficult to implement such processes in mass production environments without slowing down the fabrication or causing scratches and affecting the cleanliness due to contact application.
Another example of existing methods is described in Patent Literature 2, where time-correlated single photon counting methods or a streak camera method is used to obtain the carrier lifetime of a material. In the single photon counting method, the emission decay trace is measured directly however the time required to obtain the histogram is too long to be practically applied to the full wafer area. Whereas the streak camera method is forbiddingly costly.